I was under the impression it did not but some charges have recently popped up on my acct...
Mobile IM only incurs a MB usage charge when initially downloading the application. For signing in/out, refreshing contact list it will not charge you MB.
Also keep in mind any messages sent/received are counted the same as text messages and is considered an "out of network" message.
If you haven't downloaded the IM app recently then your MB charges are coming from somewhere else.
It costs $1.99 for the first time you use it..